I swear, if and when my Wikipedia Solar System project is finally finished, I will give each and every one of you a big hug for all the help you've given me in the close-on three years I've been slogging through it. But for now I'm still stuck.
This is a quote from the May issue of Scientific American, about the formation of planets. This is discussing the formation of a gas giant like Jupiter:
Quote:
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The planet [ie Jupiter] stabilises when it becomes massive enough to turn type I migration on its head. Instead of the disc shifting the orbit of the planet, the planet shifts the orbit of of gas in the disc. Gas interior to the planet's orbit revolves faster than the planet, so the planet's gravity tends to hold it back, causing it to fall toward the star—that is, away from the planet. Gas exterior to the planet's orbit revolves slower, so the planet tends to speed it up, causing it to move outward—again, away from the planet. Thus the planet opens up a gap in the disc and cuts off the supply of raw material. The gas tries to repopulate the gap, but computer simulations indicate that the planet wins the struggle if its mass exceeds about one Jupiter mass at 5 AU.
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I don't get this. How can gravity cause something to move
away from something else?
Also, I can't get the whole "nebula" thing straight in my head. Should the giant molecular cloud be referred to as a nebula? Or should that term only be applied to the small fragment (the pre-solar nebula) that became our Sun? To make things even more confusing, some articles refer to the pre-solar nebula as the "solar nebula", while others give that name to the disc of gas and dust that formed the planets.
I don't get it.